Fig. 1 shows the geography of the Darwin area. In the late pre-monsoon period there is a diurnal cycle of intense convection over the Tiwi Islands (Keenan et al., 1990; Keenan and Carbone, 1992; Carbone et al, 1999; Wilson et al., 1999). This phenomenon is driven by the sea breeze circulation and is so regular as to have earned the local nickname of "Hector". Hector is generally initiated by interaction of convectively-driven cold pools and see breeze fronts and builds into a bundle of intense thunderstorms reaching the tropopause in mid-afternoon, then finally moving off-shore to the west of Bathurst Is. where it dissipates by mid-evening. Updrafts as strong as 40 m/s and cloud tops to 20 km (more typically 17-18 km) have been observed during Hector. The top panel of Fig. 2 displays afternoon radar and surface meteorological observations on a particular day in late November 1995, and shows the organized nature of the convection and surface flow.

The key lower stratospheric measurements will be made from the WB-57 in repeated overflights of the convection in its developing, mature and decaying stages. Both in situ observations of winds and temperature and remote-sensing microwave observations of temperature above and below the plane will be taken. Such observations over tropical convection have been made from the NASA ER-2 on occasion. Fig. 3 shows an example from an overflight of the ITCZ during the recent NASA Stratospheric Tracers of Atmospheric Transport (STRAT) campaign. Shown are in situ vertical wind measurements from the NASA Meteorological Measurement System (MMS), temperature profiles determined from the NASA Microwave Temperature Profiler (MTP) measurements and cloud brightness temperatures measured by a downward-looking radiometer. The results show a clear enhancement of the vertical wind variance over the region of low brightness temperatures, which is the locus of a mature convective system. The data also reveal the richness of the horizontal wavenumber spectrum, with vertical wind variance peaking in the 5-10 km wavelength range, and temperature variance more apparent at wavelengths longer than about 50 km. Dominant vertical wavelengths for the latter were estimated at about 5.5 km. Fig. 3 and other previous work (Pfister et al., 1993) from the Stratosphere-Troposphere Exchange Project (STEP) have demonstrated the ability of aircraft-based observations to capture particular mesoscale gravity wave phenomena. However, the results illustrated in Fig. 3 also reveal the limitation of currently available data sets. First, since the focus of the STRAT program was not on gravity waves, there are only two (fortuitous) overflights, 3 hours apart, of a convective system with a characteristic variation time scale (as seen by half-hourly satellite photos) of perhaps 1-2 hours. One feature of the observations was that there was no evidence of the large vertical wavelength (~10 km) gravity waves generally seen in limited-area models of explicitly-resolved convection. However, this may be entirely due to the inability to observe the system throughout its entire evolution, especially since these large vertical wavelength waves propagate upward quite rapidly. Second, there is no auxiliary data to obtain information about the interior evolution of the convective system. ETCE will allow a much more systematic investigation of the gravity wave field in the lower stratosphere and its relation to tropospheric conditions. Ozone measurements near the tropopause, planned for the chemistry component of ETCE, provide a useful tracer in the lower stratosphere for measuring gravity vertical parcel displacements along the flight path. From these data, the characteristic scales of the gravity waves triggered by the convection can be inferred. The planned radiosonde observations will be important for characterizing the longer horizontal wavelength and low-frequency gravity waves associated with the storm.